What it does
Bacteria and yeast multiply by splitting in two. When it is warm and there is food and water, a microbe can double every 20 minutes or so, so 100 microbes become thousands in a few hours and millions by the next day. This page shows that growth on a dish of dots and on a graph. Change the temperature, add salt or sugar, and move the time slider to see how each one speeds growth up, slows it down or stops it. Heat above about 55 °C kills microbes instead, and boiling kills them in a minute or two. Choose yeast in dough to see the dough puff up with carbon dioxide gas, or curd bacteria to see milk turn sour as its pH falls.
It follows the school topics Class 8 Science, the invisible living world (microorganisms, useful and harmful microbes, food preservation) and the exponential growth that comes up again in Class 9 to 12 maths and biology. Look at microbes themselves in the virtual microscope, see why salt and sugar draw water out of cells in the osmosis lab, and compare growth with decay in radioactive decay dice.
How to use it
- Pick a microbe: bacteria in cooked food, curd bacteria in milk, or yeast in dough.
- Press a ready-made setting (Fridge, Warm kitchen, Pickle, Jam, Boiling) or move the sliders yourself.
- Move the time slider (up to 12 hours). The dish fills with dots as the count grows; each extra dot means more microbes on a “times ten” scale.
- Read the count, the doubling time and the sum N = N₀ × 2^(t ÷ td) under the dish.
The Level switch (Class 9 and up) draws the graph on a log scale (steady doubling becomes a straight line) and adds the number of doublings and the growth rate k = ln 2 ÷ doubling time.
Worked example
Bacteria in cooked food at 30 °C double about every 24.7 minutes in this model. After 4 hours (240 minutes) that is 240 ÷ 24.7 = 9.7 doublings, so 100 × 2^9.7 ≈ 84,000 microbes; the page shows 84,038. In the fridge at 4 °C the same food still has 100 after 4 hours. With 12 % salt (a pickle) or 65 % sugar (jam) growth stops altogether. At 100 °C every microbe is dead within the first minute.
Curd bacteria at 40 °C double every 60 minutes, so after 12 hours 100 become about 409,000 (12 doublings would give 409,600; the model slows a tiny bit as food runs out).
Check yourself
Five fresh questions on doubling, the number of doublings, time to reach 1,000, and why fridges, salt and sugar keep food. Nothing is saved.
Tips
- Doubling is fast: ten doublings multiply by about 1,000, twenty by about a million.
- A fridge slows microbes down but does not kill them. Food left out warms up and they start again.
- Real microbes differ a lot. Some grow in the cold or in salty food, and some make toxins that boiling does not destroy.
FAQ
Can I use this to decide if food is safe? No. It is a teaching model with made-up numbers. Follow food safety advice from your local health authority.
Why does growth level off? Food and space run out, and waste builds up. The model levels off near 1,000 million.
Is this from NCERT? No. It is an independent study aid that follows the same school topics, with its own model and numbers.
Where the numbers come from
- Exponential growth by doubling, N = N₀ × 2^(t/td) = N₀ e^(kt) with k = ln 2 / td, and logistic levelling off: standard school biology and maths.
- Temperature effect: the cardinal temperature model with inflection, Rosso L., Lobry J. R., Flandrois J. P. (1993), Journal of Theoretical Biology 162, 447–463 (the formula only). Heat death with a decimal reduction time and z-value: standard food science. All temperatures, doubling times and limits on the page are rounded, made-up values for a typical microbe, not data for a real species.
- Checked against an independent Python implementation of the same model, written for the tests.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.